Why is interstitial hydrostatic pressure 0

The effects of arterial and venous pressures and resistances on P C are summarized in the following relationship:. The above expression is derived from a simple model comprised of a series-coupled pre- and postcapillary resistance. In many tissues, the post-to-precapillary resistance ratio is about 0. When this ratio is 0. If this ratio increases, as occurs with arteriolar vasodilation, then arterial pressure has a greater influence on capillary pressure, which rises.

Conversely, arteriolar constriction decreases this ratio and decreases capillary pressure. This hydrostatic pressure is determined by the interstitial fluid volume and the compliance of the tissue interstitium, which is defined as the change in volume divided by the change in pressure. The more fluid that filters into the interstitium, the greater the volume of the interstitial space V i and the hydrostatic pressure within that space P i.

In some organs, the interstitial compliance is low, which means that small increases in interstitial volume lead to large increases in pressure.

Examples of this include the brain and kidney, which are encased by rigid bone brain or by a capsule kidney. In contrast, soft tissues such as skin, muscle and lung have a high compliance and therefore the interstitial space can undergo a large expansion with a relatively small increase in pressure. As interstitial volume increases, interstitial pressure increases, which can limit the amount of filtration into the interstitium because this pressure opposes the capillary hydrostatic pressure.

In other words, as the hydrostatic pressure gradient P C - P i decreases owing to the rise in interstitial pressure, fluid filtration will be attenuated. However, large increases in tissue interstitial pressure can lead to tissue damage and cellular death.

Lymph may be thought of as recycled blood plasma. Seek additional content for more detail on the lymphatic system. Watch this video to explore capillaries and how they function in the body. Capillaries are never more than micrometers away. What is the main component of interstitial fluid? Small molecules can cross into and out of capillaries via simple or facilitated diffusion. Some large molecules can cross in vesicles or through clefts, fenestrations, or gaps between cells in capillary walls.

However, the bulk flow of capillary and tissue fluid occurs via filtration and reabsorption. Filtration, the movement of fluid out of the capillaries, is driven by the CHP. Reabsorption, the influx of tissue fluid into the capillaries, is driven by the BCOP. Filtration predominates in the arterial end of the capillary; in the middle section, the opposing pressures are virtually identical so there is no net exchange, whereas reabsorption predominates at the venule end of the capillary.

The hydrostatic and colloid osmotic pressures in the interstitial fluid are negligible in healthy circumstances. Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Search for:. Capillary Exchange Learning Objectives By the end of this section, you will be able to: Identify the primary mechanisms of capillary exchange Distinguish between capillary hydrostatic pressure and blood colloid osmotic pressure, explaining the contribution of each to net filtration pressure Compare filtration and reabsorption Explain the fate of fluid that is not reabsorbed from the tissues into the vascular capillaries.

Practice Question Watch this video to explore capillaries and how they function in the body. Show Answer Water. Critical Thinking Questions A patient arrives at the emergency department with dangerously low blood pressure. True or false? The plasma proteins suspended in blood cross the capillary cell membrane and enter the tissue fluid via facilitated diffusion. Explain your thinking. Thus, even at the arterial end of the capillary bed, the net filtration pressure would be below 10 mm Hg, and an abnormally reduced level of filtration would occur.

In fact, reabsorption might begin to occur by the midpoint of the capillary bed. The plasma proteins suspended in blood cannot cross the semipermeable capillary cell membrane, and so they remain in the plasma within the vessel, where they account for the blood colloid osmotic pressure. Licenses and Attributions. CC licensed content, Shared previously.

The movement of materials across the wall is dependent on pressure and is bi-directional depending on the net filtration pressure derived from the four Starling forces that modulate capillary dynamics.

The net filtration pressure derived from the sum of the four forces described above determines the fluid flow into or out of the capillary. Movement from the bloodstream into the interstitium is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure. Alternatively, movement from the interstitium into the bloodstream is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.

Capillary Dynamics : Oncotic pressure exerted by proteins in blood plasma tends to pull water into the circulatory system. Transcytosis, or vesicle transport, is one of three mechanisms that facilitate capillary exchange, along with diffusion and bulk flow.

Substances are transported through the endothelial cells themselves within vesicles. This mechanism is mainly used by large molecules, typically lipid-insoluble preventing the use of other transport mechanisms. Vesicles are capable of merging, allowing for their contents to mix, and can be transported directly to specific organs or tissues. Due to the function of transcytosis, it can be a convenient mechanism by which pathogens can invade a tissue.

Transcytosis has been shown to be critical to the entry of Cronobacter sakazakii across the intestinal epithelium and the blood-brain barrier. Listeria monocytogenes has been shown to enter the intestinal lumen via transcytosis across goblet cells.

Shiga toxin secreted by entero-hemorrhagic E. These examples illustrate that transcytosis is vital to the process of pathogenesis for a variety of infectious agents. Pharmaceutical companies are currently exploring the use of transcytosis as a mechanism for transporting therapeutic drugs across the human blood-brain barrier.

Capillary fluid movement occurs as a result of diffusion colloid osmotic pressure , transcytosis, and filtration. Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis. Bulk flow is used by small, lipid-insoluble solutes in water to cross the the capillary wall and is dependent on the physical characteristics of the capillary.

Continuous capillaries have a tight structure reducing bulk flow. Fenestrated capillaries permit a larger amount of flow and discontinuous capillaries allow the largest amount of flow.

The movement of materials across the capillary wall is dependent on pressure and is bidirectional depending on the net filtration pressure derived from the four Starling forces. When moving from the bloodstream into the interstitium, bulk flow is termed filtration, which is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure.

When moving from the interstitium into the bloodstream, the process is termed reabsorption and is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.